Volume 20 - Materials Selection and Design Part 8 potx

Fig. 20 Mechanical models and typical behavior. (a) Ideal Hookean solid ( = E ; spring model; elastic response). (b) Ideal viscous Newtonian liquid ( = ; dashpot model). (c) Maxwell's mechanical model for a viscoelastic material. (d) Voigt's mechanical model for a viscoelastic material. Source: Ref 29 Application of a deforming force (i.e., pulling) on the spring results in an immediate stretching and thus an immediate strain. Once the force is released, the spring immediately recovers its initial length. Pulling with twice the force results linearly in twice the strain. The case of the dashpot, however, is significantly different. When the "piston" has a force applied to it, it slowly starts to move (no instant displacement as in the case of the spring), and when the force is released, the dashpot stays in its new conformation. Once a force causes an ideal viscous polymer melt to flow, it remains in its new position. Two models, combining the spring and the dashpot either in series or parallel, have been developed that attempt to better describe real polymer flow behavior. These models, Maxwell and Voigt, are named after their creators and are shown in Fig. 20(c) and 20(d). Figure 21, very similar to Fig. 15, shows which mechanical analogs model different regions of the log modulus versus temperature curve. The behavior shown in the Voigt model helps to explain the action known as creep. Creep occurs when, under a static load for extended periods of time, increased strain levels slowly develop, as in the case of a refrigerator that after many years distorts a linoleum floor. The Maxwell model describes stress relaxation, which occurs when polymers are subjected to a constant strain environment. Over time, the molecules relax and orient themselves to the strained position, thereby relieving stress. This occurs in applications such as threaded metal inserts into plastic parts and threaded plastic bottle caps. Fig. 21 Thermal dependence of elastic modulus for polys tyrene. (a) Glassy region corresponding to Hookean solid behavior. (b) Leathery region corresponding to Voigt model behavior. (c) Rubbery plateau region corresponding to Maxwell model behavior. (d) Liquid flow region corresponding to Newtonian liquid behav ior. Source: Ref 30 References cited in this section 29. M.M. McKelvey, Polymer Processing, John Wiley & Sons, 1962, p 26, 30 30. J.M.G. Cowie, Polymers: Chemistry & Physics of Modern Materials, 2nd ed., Blackie Academic and Professional, 1991, p 248 Effects of Composition, Processing, and Structure on Properties of Engineering Plastics A M.M. Baker and C.M.F. Barry, University of Massachusetts Lowell Properties of Engineering Plastics and Commodity Plastics Engineering plastics generally offer higher moduli and elevated-service temperatures compared to the lower-cost, high- volume, commodity plastics such as PE, PP, and PVC. These improved properties are due to chemical substituents, inherently rigid backbones, and the presence of secondary attractive forces as discussed earlier in this article. Engineering thermoplastics (e.g., POM, PC, PET, and polyether-imide, or PEI) are polymerized from more expensive raw materials, and their processing requires higher energy input compared to that of commodity plastics, which is why the engineering thermoplastics are more expensive. Structures of Commodity Plastics. It is interesting to note the T m elevation of HDPE from LDPE. The effect of the branched structure on density and morphology enables the high-density version to form more tightly packed crystalline regions that require more thermal energy to overcome the cohesive forces keeping the plastic from melting. Substituting a methyl group in place of a hydrogen, in the case of PP, increases T m and tensile strength further above that of HDPE. In this case, steric hindrance due to the additional size of the methyl group stiffens the chain and restricts rotation. The substitution of a large and highly electronegative chlorine atom in PVC prevents crystallization and also increases the onset of T g , both due to steric hindrance effects and to the attractive polar forces generated. Polar attractive forces are so extensive that the tensile strength can be seen to increase to 55 MPa. Polystyrene is amorphous and transparent due to the atactic positioning of the pendant phenyl group, whose randomness destroys crystallinity. The tensile strength of PS is less than that of PVC due to the lack of the highly polar pendant group. Structures of Engineering Plastics. Phenylene and other ring structures (Table 1) attached directly into the backbone often stiffen the polymer significantly, imparting elevated-thermal properties and higher mechanical properties such as increased strength. Polyoxymethylene is essentially PE with an ether substitution, but it has a much higher T m (200 °C versus 135 °C for HDPE) because of its polarity. Both of these features promote a highly crystalline morphology. The high dimensional stability, good friction and abrasion characteristics, and ease of processing of this polymer make it a popular engineering plastic for precision parts. Polycarbonate has an extended resonating structure because of the carbonate linkage. It has such a stiff backbone that crystallization is impeded, and the resultant amorphous structure is transparent, much like PET. Physical properties of PET, however, depend strongly both on its degree of crystallinity, which is governed by degree of orientation imparted during processing, and on its annealing history. The high strength, ease of processing, and clarity of PET make it ideal for soda bottles and polyester fibers. Polycarbonate has high strength, stiffness, hardness, and toughness over a range of -150 to 135 °C and can be reinforced with glass fibers to extend elevated-temperature mechanical properties. The high impact strength of high-MW PC makes it suitable for applications such as motorcycle helmets. The carbonate linkage of PC causes a susceptibility to stress cracking. Polyetherimide has both imide groups and flexible ether groups, resulting in high mechanical properties but with enough flexibility to allow processing. Its highly aromatic (presence of benzene rings) structure allows it to be used for specialty applications. Polyetheretherketone (PEEK), PPO, and PPS also rely on backbone benzene rings to yield high mechanical properties at elevated temperatures. Both sulfur and oxygen are electronegative atoms, creating dipole moments that promote intermolecular attractions and thus favorably affect elevated-temperature properties. While the composition of thermoset plastics vary widely, the three-dimensional structure produced by cross-linking prevents melting and hinders creep. Overall properties such as stiffness and strength are determined by the flexibility of the polymer structure and the number of cross-links (cross-link density). Because epoxies, phenolics, and melamine formaldehyde contain aromatic rings, they are typically rigid and hard. Epoxies are used for adhesives, assorted electronics applications, sporting goods such as skis and hockey sticks, and prototype tooling for injection molding and thermoforming. Melamine formaldehyde is easily colored and so is often found in household and kitchen equipment, electronic housings, and switches. In contrast, phenolics are naturally dark colored and are limited to electronic and related applications where aesthetics are less important. Silicones with their flexible ether linkages are softer and often used as caulking and gasket materials. Thermoset polyurethanes vary widely from flexible to relatively rigid depending on the chemical structure between urethane groups. Unsaturated polyesters are used for potting and encapsulating compounds for electronics and in glass-fiber-reinforced molding compounds. This discussion of the major commodity and engineering plastics is by no means complete. It is meant rather to include concepts touched on earlier in evaluating structures in relation to their resultant properties. Effects of Composition, Processing, and Structure on Properties of Engineering Plastics A M.M. Baker and C.M.F. Barry, University of Massachusetts Lowell Electrical Properties Volume and/or surface resistivity, the dielectric constant, dissipation factor, dielectric strength, and arc or tracking resistance are considered important electrical properties for design. These properties relate to structural considerations such as polarity, molecular flexibility, and the presence of ionic impurities, which may result from the polymerization process, contaminants, or plasticizing additives. Table 10 shows some typical electrical property values for selected plastic materials. Table 10 Electrical properties of selected plastics Dielectric constant Dissipation factor Plastic Surface resistivity, Volume resistivity, · cm Dielectric strength, kV/mm At 50 Hz At 10 6 Hz At 50 Hz At 10 6 Hz LDPE 10 13 >10 16 >70 2.3 2.3 2 × 10 -4 2 × 10 -4 PTFE 10 17 >10 18 60-80 2.1 2.1 2 × 10 -4 2 × 10 -4 PS 10 14 . . . . . . 2.6 . . . 0.5 × 10 -4 2.5 × 10 -4 PMMA 5 × 10 13 >10 15 30 3.7 2.6 0.060 0.015 PVC . . . >10 15 20-40 3.5 2.7 0.003 0.002 Plasticized PVC . . . 10 15 28 6.9 3.6 . . . . . . POM 10 13 10 15 70 . . . 3.7 0.0015 0.0055 Nylon 6/6 . . . 10 15 (dry) 10 11 (wet) 40 (dry) 4.0 (dry) 6.0 (wet) 3.4 0.02 (dry) 0.20 (wet) . . . PET 6 × 10 14 2 × 10 14 60 3.4 3.2 0.002 0.021 PBT 5 × 10 13 5 × 10 13 >45 3.0 2.8 0.001 0.017 PC >10 15 >10 16 >80 3.0 2.9 0.900 11 Modified PPO 10 14 >10 15 22 2.7 2.6 4 × 10 -4 9 × 10 -4 PAI 5 × 10 18 2 × 10 15 23 . . . 3.9 . . . 0.030 PEI . . . 7 × 10 15 24 3.15 3.05 0.0015 0.0064 PSU 3 × 10 16 5 × 10 16 20 3.15 3.10 0.001 0.005 PEEK . . . 5 × 10 16 19 3.20 . . . 0.003 . . . Source: Ref 4 Volume resistivity is a measure of the resistance of an insulator to conduction of current. Most neat polymers have a very high resistance to flow of direct current, usually 10 15 to 10 20 · cm compared to 10 -6 · cm for copper. Electrical conductivity in normally insulating polymers results from the migration of ionic impurities and is affected by the mobility of these ionic species. Generally, plasticizers with their increased mobility and high relative concentration of end groups reduce resistivity and therefore increase electrical conductivity. Because absorption of water increases the mobility of ionic species, this also reduces volume resistivity. Thus, the volume resistivity of nylon 6/6 is reduced by four decades when the polymer absorbs water at ambient conditions. Addition of antistatic agents decrease surface resistivity because the polar additives migrate to the surface of the polymer and absorb humidity. In contrast, conductive fillers, such as carbon black powders and aluminum flake, can form three-dimensional pathways for conduction through insulating polymer matrices. Finally, highly conjugated polymers such as polyacetylene and polyaniline provide sufficient electron movement to reach semiconductor conductivity. For full conductivity, they rely on dopants. Dielectric Constant and Dissipation Factor. In the presence of an electric field, polymer molecules will attempt to align in that field. The dielectric constant (or permittivity), or ', is a measure of this polarization. While the dielectric constant varies from 1 for a vacuum (where nothing can align) to 80 for water, the values for polymeters (shown in Table 10) are generally so low that most polymers are insulators. The dielectric constant also varies with temperature, rate or frequency of measurement, polymer structure and morphology, and the presence of other materials in the plastic. The dielectric constant of polymers typically peaks at the major thermal transition temperature (T g and/or T m ) and then decreases because of random thermal motions in the melt. As shown in Fig. 22(a), the dielectric constant decreases abruptly as frequency increases.This occurs between 1 Hz and 1 MHz and is a result of the inability of the dipoles to align with the high-frequency electric fields. The dielectric loss, '', is a measure of the energy lost to internal motions of the material, and as shown in Fig. 22(b), peaks where the dielectric constant changes abruptly. The dissipation factor, tan , which is given by: (Eq 8) is a measure of the internal heating of plastics. Thus, little heating should occur in insulators (tan < 10 -3 ), whereas high- frequency welding necessitates that tan be much greater (Ref 32). Fig. 22 Frequency dependence of the (a) dielectric constant and (b) dielectric loss. Source: Ref 31 Because polymer molecules are typically too long and entangled to align in electric fields, the dielectric constant usually arises from shifting of the electron shell of the polymer and/or alignment of its dipoles in the field. For nonpolar polymers, such as PTFE and PE, only electron polarization occurs and the dielectric constant can be approximated by: = n 2 (Eq 9) where n is the optical refractive index of the polymer. These values vary little with frequency, and changes occurring with increased temperatures are caused by changes in free volume of the polymer. In contrast, the dielectric constants of polar polymers, such as PVC and PMMA, are greater than n 2 and change substantially with temperature and frequency. Backbone flexibility or ease of rotation of polar side groups allows some polymers to orient quickly and easily. If the electric field alternates slowly enough, the molecule may be able to align or orient in the field depending upon its flexibility and mobility. Consequently, relatively flexible polymers, such as PVC and PMMA, exhibit greater decreases in dielectric constant with increased frequency than polymers, such as PEI and PSU, that have rigid backbones. The additional free volume and mobility of the plasticized PVC allows the molecules to align with minimal delay; as shown in Table 10, this doubles the dielectric constant at low frequencies. Dielectric Strength. As the electric field applied to a plastic is increased, the polymer will eventually break down due to the formation of a conductive carbon track through the plastic. The voltage at which this occurs is the breakdown voltage, and the dielectric strength is this voltage divided by the thickness of the plastic. The dielectric strength decreases with the thickness of the insulator because this prevents loss of internal heat to the environment. Dielectric strength is increased by the absence of flaws. Arc Resistance. In contrast to the dielectric strength, arc resistance is the ability of a polymer to resist forming a carbon tracking on the surface of the polymer sample. Because these tracks usually emanate from impurities surrounding electrical connections, arc resistance is measured by the track times. Polymers, such as PC, PS, PVC, and epoxies (which have aromatic rings, easily oxidized pendant groups, or high surface energies), are prone to tracking (Ref 33) and exhibit typical track times of 10 to 150 s (Ref 34). However, polyesters may have better tracking resistance than phenolics because of the heteroatomic backbone that disrupts the carbon track. Nonpolar aliphatic compounds or those with strongly bound pendant groups usually have better arc resistance; thus, the tracking times for PTFE, PP, PMMA, and PE are greater than 1000 s (Ref 33). References cited in this section 4. H. Dominghaus, Plastics for Engineers: Materials, Properties, and Applications, Hanser Publishers, 1988 31. R.D. Deanin, Polymer Structure, Properties and Applications, Cahners Books, 1972, p 109 32. W. Michaeli, Plastics Processing, an Introduction, Hanser Publishing, 1992, p 59 33. C.C. Ku and R. Liepins, Electrical Properties of Polymers: Chemical Principles, Hanser Publishers, 1987, p 181-182 34. A.B. Strong, Plastics: Materials and Processing, Prentice-Hall, 1996, p 144 Effects of Composition, Processing, and Structure on Properties of Engineering Plastics A M.M. Baker and C.M.F. Barry, University of Massachusetts Lowell Optical Properties Transparency, opacity, haze, and color are all important characteristics of plastics. Optical clarity is achieved when light is able to pass relatively unimpeded through a polymer sample. This is usually defined by the refractive index, n, which is shown in Fig. 23 and given by: (Eq 10) where is the angle of incident light and is the angle of refracted light. While n for most polymers is 1.40 to 1.70, it increases with the density of the polymer and varies with temperature. In order for a material to be clear, light has to be transmitted with minimal refraction. Unstressed, homogeneous, amorphous polymers, such as PS, PMMA, and PC, exhibit a single refractive index and thus are optically clear. However, when these polymers are severely oriented, and therefore stressed, the areas with different refractive indices produce birefringence in the molded products. Because amorphous, but heterogeneous, systems, such as the immiscible polymer blends ABS and HIPS, typically exhibit a refractive index for each polymer phase, they are usually opaque or translucent. Semicrystalline polymers, such as HDPE and nylon-6/6, effectively have two phases, the amorphous and crystalline regions. Consequently, semicrystalline polymers are usually not transparent. Finally, introduction of any nonpolymeric phases, such as fillers or fibers, into the plastic material induces opacity because these phases have their own refractive indices. Fig. 23 Light refracted by a plastic sample Optical clarity can also be controlled by polymerization techniques. When the refractive indices of multiphase systems are matched, these plastics can be optically clear, but usually only over narrow temperature ranges. Neat poly-(4-methyl-1- pentene) (TPX) is clear because the bulky side chains produce similar densities (0.83 g/cm 3 ), and thus similar refractive indices, in the amorphous and crystalline regions of the polymer. Matching of refractive indices of PVC and its impact modifier is often used in transparent films for food packaging. Domains (second phases) that are smaller than the 400 to 700 nm wavelengths of visible light will not scatter visible light, and thus do not reduce clarity. In impact-modified polymers, the minor rubbery phase is usually dispersed as particles with diameters greater than 400 nm, so most of them are opaque. However, when the domains have diameters less than 400 nm or when the two phases form concentric rings whose width is too narrow to scatter visible light, the blends are clear. When crystals are smaller than the wavelength of visible light, they will also not scatter light and the plastic will be optically clear or translucent. These crystal sizes can be controlled by quenching, use of nucleating agents, stretching, and copolymerization. In quenching, the plastic melt is rapidly cooled below the transition temperature of the polymer. The resultant reduction in thermal mobility of the polymer molecules limits crystal growth because the molecules are not able to form ordered structures. While quenching is more easily accomplished with thin parts and films, nucleating agents can reduce crystal size in a wider range of parts. The agents are small particles at which the crystallization process can begin. Consequently, many such sites competing for polymer chains will reduce the average crystal size. Stretching also promotes clarity because the mechanical stretching can break up large crystals, and the resultant thinner films are more liable to transmit light without refraction. Finally, copolymerization can reduce the regularity of the polymer structure enough to inhibit formation of large crystals. As discussed earlier, structural regularity is required of a polymer is to pack into tightly order crystallites, and randomization of the structure results in smaller areas capable of being packed together. The surface character of processed parts also controls optical properties. Smooth surfaces reflect and transmit light at limited angles, whereas rough surfaces scatter the light. Consequently, smooth surfaces produce clear and glossy products while rough surfaces appear dull and hazy. Because surface character is usually controlled by processing, it is discussed in the next section. Unmodified polymers are usually clear to yellowish in color. Other colors are produced by dispersing pigments or dyes uniformly within the plastic. Poor dispersion can produce the marbled or speckled appearances favored for cosmetic cases. However, degradation of polymers will produce yellowing or browning of the plastic. Polymers such as PVC, which are particularly subject to degradation, are also discussed in the section "Processing" in this article. Effects of Composition, Processing, and Structure on Properties of Engineering Plastics A M.M. Baker and C.M.F. Barry, University of Massachusetts Lowell Chemical Properties Solubility is the ease with which polymer chains go into solution and is a measure of the attraction of the polymer to solvent molecules. The old adage of "like dissolves like" can be explained by considering the balance of forces that occur during dissolution of the polymer. Solubility is determined by the relative attraction of polymer chains for other polymer chains and polymer chains for solvent molecules. If the polymer-solvent interactions are strong enough to overcome polymer-polymer interactions, dissolution occurs; otherwise, the polymer remains insoluble. Swelling can be considered as partial solubility because the solvent molecules penetrate the polymer, but they cannot completely separate the chains. When solvents and polymers have similar polarities, the polymer will dissolve in or be swollen by the solvent. Because longer chains are more entangled, higher MW hinders dissolution. Semicrystalline polymers are much harder to dissolve than similar amorphous materials. The tightly packed crystalline regions are not easily penetrated because the solvent molecules must overcome the intermolecular attractions. Elevated temperatures, which increase the mobility of solvent molecules and polymer chains, facilitate dissolution. The presence of cross-links completely prevent dissolution, and such polymers merely swell in solvents. Plasticizers must be soluble in the polymer to prevent migration to the surface (blooming) and extraction by solvents. Consequently, the relatively expensive primary plasticizers for PVC closely match the solubility of the polymer, while less expensive secondary plasticizers are less compatible with the PVC. Permeability is a measure of the ease with which molecules diffuse through a polymer sample. The low densities of polymers compared with metals and ceramics allow enhanced permeation of species such as water, oxygen, and carbon dioxide. If there are strong interactions between the polymer and the migrating species, adsorption will be high, but permeation may be low as the migrating species is delayed from diffusing. For example, the electronegative chlorine atoms substitution in polyvinylidene chloride (PVDC) enhances adsorption of oxygen, nitrogen, carbon dioxide, and water while its tightly packed chain arrangement restricts diffusion of these species. Thus, PVDC films (commonly used as plastic wrap) are extremely valuable in food packaging operations. As shown in Fig. 24, permeability can also be inhibited by the addition of platelike fillers, which increase the distance that water must travel in order to pass completely through the plastic. Fig. 24 Barrier pigment effect. Water passes relatively unobstructed through a polymer with spherical additives (a), but must travel around platelike fillers (b). Source: Ref 35 Environmental stress cracking occurs when a stressed plastic part is exposed to a weak solvent, often moisture. The stress imparts strain to the polymer, which allows the solvent to penetrate and either extract small molecules of low n , or to plasticize and weaken the polymer. The stress then causes fracture at these weak areas. Polymers which are exposed to UV light are particularly susceptible to environmental stress cracking. Resistance is enhanced when the permeability of the polymer to water is low. Reference cited in this section 35. M.J. Austin, Inorganic Anti-Corrosive Pigments, Paint and Coating Testing Manual, J.V. Koleste, Ed., ASTM, 1995, p 239 Effects of Composition, Processing, and Structure on Properties of Engineering Plastics A M.M. Baker and C.M.F. Barry, University of Massachusetts Lowell Processing Most thermoplastic processing operations involve heating, forming, and then cooling the polymer into the desired shape. This section briefly outlines the most common plastics manufacturing processes. The factors that must be considered [...]... 40 PP 0.0 1-0 .03 200 -2 60 0.25 100 PMMA 0.1 0-0 .40 24 0-2 60 0.40 40 PVC, rigid 0.0 4-0 .40 140 -2 00 0 .20 20 ABS 0 .2 0- 0.45 200 -2 60 0.30 50 POM 0.2 5-0 .40 19 0-2 30 0.45 40 Nylon 6/6 1.0 0-2 .80 27 0- 320 0.50 60 PET 0.1 0-0 .20 28 0-3 10 0.50 PBT 0.0 8- 0 .09 22 0- 260 0.40 50 PC 0.15 28 0- 320 0.50 40 PS 0.30 31 0-3 40 0.50 50 Source: Ref 8, 39 The combination of temperature and shear can also degrade plastics The long entangled... 0.01 5-0 .040 0.00 2-0 .004 PP 0.01 0-0 .025 0.00 2-0 .005 PS 0.00 4-0 .007 ABS 0.00 4-0 .009 0.00 2-0 .003 POM 0.01 8- 0 .025 0.00 3-0 .009 Nylon 6/6 0.00 7-0 .0 18 0.003 PET 0. 02 0- 0.025 0.00 2-0 .009 PBT 0.00 9-0 .022 0.00 2-0 .0 08 PC 0.00 5-0 .007 0.00 1-0 .002 PSU 0.007 0.00 1-0 .003 PPS 0.00 6-0 .014 0.00 2-0 .005 Source: Ref 39 Crystallinity can also vary through the thickness of a part with the rapidly cooled outside surfaces and. .. 2.75 0.400 400 60 Boron nitride, fibers 1. 8- 2 .0 0. 3-1 .4 0.04 5-0 .20 2 8- 8 0 4-1 0 Silicon nitride, whiskers 3.2 5-7 0.7 5-1 .0 35 0-3 80 5 0-5 5 Carbon whiskers >2.0 700 100 Carbon fibers 1. 8- 2 .0 2-3 0.2 9-0 .44 23 0-5 50 3 5 -8 0 Carbon-matrix reinforcements Source: Ref 4 (a) Slurry-spun continuous fiber (b) Uncoated (c) Silicon carbide-coated Fig 2 Mechanical properties and service temperatures for selected reinforcement... 400 58 Alumina powder pressed, sintered, and formed into fiber . 10 3 s -1 HDPE <0.01 18 0-2 40 0 .20 40 PP 0.0 1-0 .03 200 -2 60 0.25 100 PMMA 0.1 0-0 .40 24 0-2 60 0.40 40 PVC, rigid 0.0 4-0 .40 140 -2 00 0 .20 20 ABS 0 .2 0- 0.45 200 -2 60 0.30 50 POM 0.2 5-0 .40. 0.2 5-0 .40 19 0-2 30 0.45 40 Nylon 6/6 1.0 0-2 .80 27 0- 320 0.50 60 PET 0.1 0-0 .20 28 0-3 10 0.50 . . . PBT 0.0 8- 0 .09 22 0- 260 0.40 50 PC 0.15 28 0- 320 0.50 40 PS 0.30 31 0-3 40 0.50 50 Source: Ref 8, 39. 0.01 5-0 .040 0.00 2-0 .004 PP 0.01 0-0 .025 0.00 2-0 .005 PS 0.00 4-0 .007 . . . ABS 0.00 4-0 .009 0.00 2-0 .003 POM 0.01 8- 0 .025 0.00 3-0 .009 Nylon 6/6 0.00 7-0 .0 18 0.003 PET 0. 02 0- 0.025